Timers and clock mechanisms for downhole use in oil well drilling and logging and in drill stem testing are well-known in the petroleum engineering art.
One of these devices which is the exemplary but nonlimiting example of the type of clock in which the present invention is used is the very well-known KPG type gauge clock. Such clocks are manufactured by Geophysical Research Corporation of Tulsa, Okla., and a similary clock is manufactured by the V. E. Kuster Company of Long Beach, Calif. Other downhole clocks utilizing a similar principle are also in common usage. Since these clocks are so generally known, only a general description is given here.
The clocks to which the present invention relates are in the form of a cylindrical mechanism several inches long and from an inch to two inches in diameter, one and one-fourth being a typical diameter. The clocks comprise a spring, usually a plurality of springs connected in tandem, an escapement mechanism and a gear train. These clocks also include means for winding the spring before each use. The springs are elongate, flat ribbon springs which upon being wound up are wrapped around a central mandrel and upon being released to be unwound cause the central mandrel to rotate. The central mandrel, acting under the torque force from the spring, rotates at a fixed rate which is determined by an escapement mechanism and drives a gear train which drives a scriber, needle or other indicator as a function of time. This type of device is, as will be readily noted, simply a basic, simple mechanical clock, but is uniquely configured to a cylindrical configuration to enable the entire mechanism to be dropped into an oil well or drill stem casing. The clock is usually associated with a pressure of temperature measuring device, or some other measuring instrument. The combined instrument records, usually on a chart, the measured pressure, temperature, etc., as a function of time, the time line being generated by the aforementioned clock. Reference is made to any of the dozens of patients, catalogs and other technical publications of the oil logging and drilling industry for more detailed descriptions of these instruments.
The springs typically utilized in these instruments must be very rugged and capable of heavy duty operation under adverse conditions. These springs may typically be formed of a flat ribbon about two feet long and from one-half to one inch in width with aperatures, etc., at the ends for connecting the spring to a mandrel and a shell inside the cylindrical clock mechanism.
While these instruments are usually quite reliable, they are not without problems in operation. One problem which has been discovered is a tendency of the spring to hang-up upon its self by reason of friction between the layers of the wound spring. This in only momentary and in many applications would be no particular problem. However, where, as in the clocks just described, it is of great importance that the torque on the mandrel and through the escapement to the gear train be uniform, even a momentary pause can result in error or erratic results. These pauses in unwinding usually last no more than a fraction of a second but may introduce uncertainty as to the total output of the measuring device.
The present invention involves the discovery of the source of the problem which has plagued the industry for many years and a solution to the problem.